Small amendment: This is the concept for the copper tubing I came up with: Take a 16 mm OD/12 mm ID pipe, cut a G1/4" thread into it, screw fitting in, done. Works very well, you just need to make sure you have a clean cut so that the fitting's O ring can seal against the pipe.

These are from a test I did to see how well it works:

What I like about the concept is that you don't get any bulky fittings, the pipes are pretty much the same diameter as the fittings. Downside is that you can't really bend 16/12 tubing, so you have to use tons of dual-rotary, dual-male threaded angles. And unfortunately, only Bitspower seems to be making these (or at least I've only found them from BP), which is why it would (will? ) be so expensive.